The human hand consists of several small bones called phalanges and metacarpals. The forearm consists of two bones, namely, the radius and the ulna. The wrist is broadly defined as the multiple articulations of the eight carpal bones (carpus) with the neighboring hand and forearm. This complex system of articulations works in unison to provide a global range of motion for the wrist joint. Motion at the wrist joint occurs between the radius and the first (or proximal) row of carpal bones, which move essentially as a single functional unit, and between the proximal carpal row and the distal carpal row of carpal bones. There is minimal motion between the bones of the distal carpal row and the metacarpal bones of the hand.
As illustrated in FIG. 1, there are eight carpal bones, namely, the scaphoid 10, lunate 12, triquetrum 14, pisiform 16, trapezium 18, trapezoid 20, capitate 22 and the hamate 24. Each carpal bone possesses a unique, highly complex three-dimensional shape. The majority of their outer surfaces consist of two types of tissue: a cartilaginous tissue permitting articulations with other bones, and fibrous tissues permitting insertions of ligaments. The arrangement of the eight carpal bones can be grossly described as consisting of two rows to form a compact, powerful unit. The proximal carpal row contains the scaphoid 10 (also called the navicular), lunate 12, and triquetrum 14. These three bones articulate proximally with the radius 40 and the triangular fibrocartilage. The ulna 50 does not articulate directly with the carpus but is separated from the triquetrum 14 by the triangular fibrocartilage, which acts as a stabilizing structure. The distal carpal row contains the trapezium 18, trapezoid 20, capitate 22, and hamate 24 and articulates distally with the five metacarpals (metacarpals bones 30, 31, 32, 33 and 34), and proximally with the three bones of the proximal carpal row. The pisiform 16 is a sesamoid bone which articulates with the triquetrum alone, and does not participate directly in carpal or global wrist motion. The scaphoid 10 acts as a connecting link between the proximal and distal carpal rows and is a critical coordinator of carpal motion.
The wrist is generally divided into five primary articulations in addition to the intercarpal joint spaces: the radiocarpal joint, the midcarpal joint (between the proximal and distal carpal rows), the large carpometacarpal joint (between the distal carpal row and the second, third, fourth and fifth metacarpals (bones 31, 32, 33 and 34), the thumb carpometacarpal joint (between the first metacarpal 30 and the trapezium 18) and the distal radio-ulnar joint (DRUJ). The midcarpal joint is the joint between the scaphoid, lunate, and triquetrum proximally, and the second row of carpal bones distally and is made up of three distinct portions: in the center the head of the capitate and the superior surface of the hamate articulate with the deep cup-shaped cavity formed by the navicular and lunate, and constitute a sort of ball-and-socket joint. On the radial side of the midcarpal joint, the trapezoid and trapezium form a concave articulating surface with the distal scaphoid. On the ulnar side, the convex hamate surface articulates with a helicoidal surface of the distal triquetrum. The midcarpal joint can thus be characterized as containing a number of convex and concave surfaces that interact with one another to provide the desired joint movements.
Radiocarpal ligaments limit motion between the radius and the carpal bones and intercarpal ligaments limit motion between neighboring carpal bones. The distal radioulnar joint is an articulation between the radius and the ulnar head, and is contained within a capsule-like structure of cartilage, synovial membrane and ligaments. A triangular fibrocartilage between the radius 40 and the carpus separates the distal radioulnar joint from the rest of the wrist.
The hand and wrist are involved in virtually every human functional activity and as such, are vulnerable to a high number of traumatic injuries, primary osteo-arthritis, and secondary degenerative disease. Examples of traumatic injuries include a break (fracture) of a carpal bone, a dislocation of all or part of the carpus, or a ligament injury between one or more of the carpal bones. Osteo-arthritis, also known as degenerative joint disease, is a process of progressive deterioration of articular cartilage and formation of new bone (osteophytes) at the joint surface. Primary osteo-arthritis is age related and associated with repetitive and/or high mechanical stress on a normal joint. Secondary osteo-arthritis is due to an underlying cause, such as trauma, inflammatory, metabolic, developmental, or connective tissue diseases. Importantly, untreated traumatic injuries of the carpus (e.g. scaphoid fractures, scapho-lunate ligament injuries) are the most frequent cause of secondary osteo-arthritis of the wrist.
There are a number of different techniques and surgical procedures for remedying either an injury and/or the effects of wrist degeneration. For example, a total joint arthroplasty is a surgical procedure that replaces the entire joint with an artificial implant (artificial joint). Wrist arthodesis (i.e., a partial or complete surgical fusion of the wrist) is typically favored by most surgeons over wrist arthroplasty for active patients, despite the limitations of motion and function associated with wrist fusion. This is due in large part to the uncertain outcome of wrist arthroplasty, which may result from implant wear, loosening and failure. Each of these procedures and the existing wrist arthroplasty devices has limitations and/or deficiencies.
The design of wrist prostheses has evolved based on clinical experience, and kinematic and biomechanical studies. This evolution (which spans more than 30 years) has generally yielded three distinct generations of total wrist arthroplasty implants. The first generation was a one-piece silicone design that had no articulating components and was made from a high-performance elastomer. The second generation used two articulating components, usually with a metal-on-polyethylene bearing, and cemented fixation into the meta-carpal canals distally and the radius proximally. The third generation implants included the Biaxial, Trispherical, and other types of prostheses. These designs attempted to improve wrist balance and prosthetic durability.
Continued problems with distal component loosening and wrist imbalance with these prostheses prompted development of a total wrist implant called the Universal by Kinetikos Medical, Inc. of San Diego, Calif. This particular prosthesis has a different method of fixation for the distal component in that it is fixed by a short central stem cemented into the capitate, and features two deep threaded osteointegrated screws are fixed into the radio and ulnar aspects of the carpus. This fixation is combined with a partial intercarpal arthrodesis to provide the potential for long-term implant survival. While this type of implant, as well as other new designs, has had some success in some patients, they continue to have problems with loosening of prosthetic fixation, and instability. Current prosthetic designs typically replace the radiocarpal joint surface rather than the midcarpal joint surface, and as such, may restrict certain highly important functional motions of the wrist.
The radial and carpal base components of current prostheses are generally made of CoCr and Ti, respectively. In these systems, the radial component is inclined 20 degrees to replicate the inclination of the articular surface of the normal distal radius. A convex ultra-high molecular weight polyethylene component fixed to the carpal base provides the distal articular surface. The shape of this component is generally elliptical. These articulating surfaces of the carpal and radial components create a dual-axis articulation that is best suited for planar motions of radial and ulnar deviation or flexion and extension. In other words, true congruency is maintained in only uniplanar rotations. If the wrist is moving through radio-ulnar deviation for instance, conjoined motions of flexion or extension will cause partial liftoff of the component, articular incongruency, uneven articular wear, implant fixation stress, and potential implant instability. A similar situation exists with planar motion in flexion-extension, wherein conjoined motion of radio-ulnar deviation will cause similar incongruency of the components. The articular concavity of the radial component (toroidal shaped) has been described as deep enough to provide immediate stability when the components are inserted under appropriate tension. Soft tissue balancing can be adjusted by varying the polyethylene thickness to limit component instability. Although conventional prosthetic designs have improved the problems associated with loosening of the distal component, their design does not fully address this problem, and other problems, including dislocation, wear, and instability.
Most sporting activities, occupational activities and many activities of daily living utilize non-planar motions of the wrist (i.e., neither pure flexion-extension or pure radio-ulnar deviation). The “dart-thrower's arc” of wrist motion is defined as a coupled or conjoined motion of flexion-extension and simultaneous radio-ulnar deviation. In throwing for instance, the hand and digits grip an object (rock, baseball, dart, javelin) and the wrist is simultaneously cocked into extension and radial deviation, which initiates the activity. The shoulder and elbow are activated to raise the object overhead. During the throwing portion, the shoulder, elbow, forearm and wrist participate to deliver the object in a smooth, accurate and coordinated sequence of radial extension to ulnar flexion. To maximally accelerate the object, the follow-through portion of wrist motion terminates with the hand in a coupled position of flexion and ulnar deviation. Procedures or diseases which impair this arc have been demonstrated to cause marked functional impairment. Recent kinematic evidence has shown a remarkable degree of uniformity of carpal motion within the dart-thrower's motion (DTM), and near absence of motion in the entire proximal row of carpal bones. The dart-thrower's arc of motion occurs almost exclusively at the midcarpal joint (the joint between the distal carpal row (triquetrum, trapezoid, capitate and hamate) and the proximal carpal row (scaphoid, lunate and trapezium). The human midcarpal joint has a unique convex and concave surface that differs distinctly from other primate midcarpal joints. Anthropologic evidence suggests that the dart-thrower's arc may be unique to humans and may have provided an evolutionary advantage for hunting, combat, and protection of offspring. Ongoing 3D motion analysis studies are demonstrating the precise motion paths of the wrist during occupational and sporting activities in normal and injured patients and will provide data to help better design rehabilitation protocols and devices to optimize this motion.
It should be emphasized that there may be different proportions of flexion-extension and radial-ulnar deviation for different activities; thus there may be several unique “dart-thrower's arcs” of wrist motion. Most functional activities begin with 10-30 degrees of wrist extension and 10-30 degrees of radial deviation, and finish in 10-40 degrees of ulnar deviation and 10-40 degrees of wrist flexion. All share in common, however, a smooth coupling of motion that progresses from some amount of wrist extension and radial deviation to some amount of wrist flexion and ulnar deviation. There are some activities (e.g. Frisbee throwing) that require a reverse dart-thrower's motion (from ulnar deviation and wrist flexion to radial deviation and wrist extension), such that the direction of motion is opposite to the dart-thrower's motion, but the coupled motion path is the same, and unique to the orthogonal axes of wrist flexion-extension and radial-ulnar deviation. Importantly, there are no functional activities that have been demonstrated to use an inverse dart-thrower's motion (i.e., from radial flexion to ulnar extension or vice-versa).
Conventional arthroplasty devices are incapable of providing and/or perfecting the above-described dart thrower's motion that is critical to performing a significant number of everyday activities.
Referring now to FIG. 2 in which x, y and z axes and motions of rotation described as radial/ulnar deviation, flexion/extension, and supination/pronation are illustrated. As shown, radial/ulnar deviation is performed about the y-axis. Rotation about the positive y-axis is defined to be radial deviation. The flexion/extension motion is performed about the x-axis, with flexion in the positive x direction. Supination/pronation, which physiologically occurs primarily between the radius and ulna, can also be described relative to the coordinate system in FIG. 2 as rotation about the z-axis, with supination in the positive z direction. Accordingly, translational motion can be described using this same coordinate system. Dorsal/ventral translation occurs along the y-axis, with dorsal being in the positive y direction. Radial/ulnar translation occurs along the x-axis, with radial translation being positive. Distal/proximal translation occurs along the z-axis, with distal translation being in the positive z direction. It should be noted that these well accepted descriptions of wrist motion direction of are based on anatomical planes, which are not necessarily the planes of motion that are the most functional or the most common. These anatomically defined motions of radial/ulnar deviation and flexion/extension are orthogonal to each other. The dart thrower's motion, which is wrist motion oblique to the orthogonal coordinate axes in FIG. 2 and consists of an arc of motion that includes combined radial extension to and from ulnar flexion, is an important functional motion of the wrist. In other words, considering the rotation axis of flexion/extension is the x axis, and the rotation axis of radial/ulnar deviation is the y axis, the rotation axis of the dart throwers' motion contains components of both the x axis and the y axis. The dart thrower's motion is used to describe any wrist motion that includes components of both flexion/extension and radial/ulnar deviation.